The present invention relates generally to an apparatus and method for simulating the effects of a fusion environment on materials and more particularly to an apparatus and method of simulating a fusion environment on first wall and blanket materials using a fast fission reactor.
The quest to tap the energy of nuclear fusion by magnetically confining an ultra hot plasma has been in progress for more than three decades. Many technological problems remain to be resolved before a fusion rector will be economically viable. One of the most important factors involves the choice of materials for first wall and blanket structures.
The first wall materials will be exposed to high neutron fluxes at energies up to 14 MeV and a potentially corrosive chemical environment. Additionally, many reactor designs call for pulsed operation which leads to cyclic stresses, temperatures and neutron fluxes. The unique radiation exposure of first wall and blanket structures generated by an operating fusion reactor creates the need for a well developed data base on the effects of such radiation on materials properties. Irradiation damages studies must be performed to determine the effects on physical and mechanical properties of the materials.
Since no prototypic fusion reactors are presently available, other techniques must be used to simulate the irradiation effects that are expected to be produced by the intense high-energy neutron flux generated in an operating fusion reactor. The simulation must generate damage on materials similar to that expected from an operating fusion reactor.
Several irradiation effects techniques for simulating fusion reactor conditions and radiation damage mechanics are discussed by R. L. Klueh et al. in ORNL-5830 (1981) which is incorporated herein by reference.
One technique is the irradiation of materials with neutrons from a fission reactor. The primary difference between the fusion reactor irradiation environment and that in the core of a fission reactor is the high-energy component of the neutron spectrum (up to 14.1 MeV) resulting from the fusion reaction compared to the average creation energy of neutrons in a fast reactor (about 2 MeV). The high energy of the fusion neutrons creates somewhat higher displacement damage per neutron interaction with a lattice atom. Displacement implies displacement of an atom from its normal lattice position. The displacement is caused by the collision of a neutron with a lattice atom. The extent of displacement damage is expressed in terms of how often an atom is displaced from its normal lattice position as displacements per atom or dpa.
In addition to the displacement damage, the neutrons in both fission and fusion reactors will also give rise to transmutation reactions which produce solid products as well as helium and hydrogen within the structural materials.
At the projected operating temperature of commercial fusion reactor structures (250.degree.-700.degree. C.) the transmuted hydrogen will have a high mobility in most materials and may readily diffuse out of the materials. However, small amounts of transmuted helium produced within the lattice can have pronounced effects on the material properties, since helium has low mobility and is essentially insoluble in metals and alloys. As a result, the helium atom affects void nucleation and swelling. Also, at elevated temperatures the collection of helium on grain boundaries leads to loss of ductility or helium embrittlement.
The neutron energy spectrum corresponding to a fusion reactor will generate much larger concentrations (approximately 100 times larger) of transmutation helium than is produced by a fast fission reactor. Because the evaluation of microstructures during irradiation depends largely on the interaction of the helium with the displacement damage, it is important that irradiation damage studies properly simulate the helium production rates that correspond to an operating fusion reactor.
Another technique used in irradiation studies is the use of accelerator based high-energy neutron sources. Because of the small irradiation volume, only a small number of specimens can be irradiated. Further, the low fluences of irradiations in current accelerator-based high-energy neutron sources preclude the formation of significant amounts of helium and displacement damage.
Another available technique for alloys that contain nickel is the use of mixed-spectrum reactors, that is, reactors which have both thermal and fast neutron spectra. Due to the interaction of nickel with the thermal neutrons, helium will be generated in concentrations similar to those produced in a fusion reactor first wall. The fast neutron flux will also cause displacement damage similar to a fusion environment. This method, however, is limited to alloys that contain nickel. Problems exist in achieving simultaneous production of helium and displacement damage at ratios similar to those produced in a fusion reactor.
Other techniques which have been proposed in order to obtain the desired helium and dpa levels are the use of stainless steel uniformly doped with .sup.10 B and the preinjection of specimens with helium prior to neutron irradiation.
Another available technique for obtaining helium in alloys with a high hydrogen solubility involves dissolving tritium in the alloy to be tested. The tritium decays to .sup.3 He with a half-life of 12 years. It has been proposed that this technique could be used to study the effect of the simultaneous helium and displacement damage production during fission-reactor irradiation of selected materials. This technique would be especially useful for niobium, vanadium and titanium alloys which have high solubility for hydrogen. The experimental procedures involve the handling of tritium which must be contained.
Andersen et al., J. Nucl. Mater. 85 and 86 (1979) 435 have proposed the use of a pressurized capsule containing .sup.3 He. Test specimens are disposed in the pressurized capsule and irradiated in a mixed-spectrum reactor. Displacement damage is obtained from the fast neutrons while the thermal neutrons produce tritium and protium. The hydrogenic species is introduced into the specimens by solution or ion implantation. The difficulty associated with this technqiue is that the ions must be implanted by energetic methods. To date this technique has not been used.
Some of the promising materials for fission reactor first wall and blanket structures are vanadium-based alloys. Vanadium based alloys exhibit several properties which are favorable for such applications. Vanadium and some selected vanadium-based alloys qualify as low activation materials. This low-activation material property is particularly important with respect to waste management. The alloys exhibit good high temperature strength and relatively low thermal stresses because of low thermal expansion and high thermal conductivity properties. The limited data available indicate that vanadium-based alloys are also highly resistant to radiation induced swelling.
Dominant concerns regarding the use of vanadium-based alloys for fusion reactor applications relate to the following properties of the alloy: its nonmetallic element interactions, fabricability issues, and sensitivity to radiation embrittlement. For vanadium and vanadium-based alloys there are no heretofor available methods for irradiation studies which simultaneously produce displacement damage and transmutation in these materials.
Therefore, in view of the above, it is an object of the present invention to provide an environment which simulates the conditions on a first wall and blanket structure of an operating fusion reactor.
It is a further object of the present invention to provide an apparatus and method which will simultaneously generate a dpa and helium production rate similar to that of an operating fusion reactor in vanadium-based alloys and other materials having a high hydrogen solubility.
It is still another object of the present invention to provide an environment which simulates a fusion environment using presently available neutron sources.
Additional objectives, advantages and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.